In the US, car tire tread depth is measured in 32nds of an inch. New tires typically come with 10/32” or 11/32” tread depths, and some truck, SUV and winter tires may have deeper tread depths than other models. A tire is legally worn-out if the tread depth is 2/32nds. Therefore, a typical tire that starts with 10/32″ of original tread depth has only 8/32″ of usable tread depth. A tire that started with 10/32″ of original tread depth and has worn off 4/32″ (down to 6/32″ of remaining tread depth) is 50% worn. Though 2/32nds of tread depth is the legal minimum, you should change your tires when they reach 4/32nds of tread depth. At tread depths less than 4/32nds, you may experience hydroplaning when driving in the rain and lose control of your vehicle.
Tread Depth Gauge
There are manual and digital tread depth gauges. I’d go with a manual one so you don’t have to worry about replacing a battery.